1. Field of the Invention
The invention relates to a device and method for degassing a suspension, and, more particularly, to a method and device for degassing a fiber suspension.
2. Description of the Related Art
In a broad sense the term “suspension” refers to white water containing the essential components of fines and ash, or clarified water containing a minimum of fines and ash content.
Methods and devices for degassing suspensions, specifically fiber stock suspensions that are utilized in paper and cardboard production, are well known to the experts in this field. Consistent with their names, the devices are intended to degas suspensions. During the degassing process other light contaminates, which would otherwise interfere with the paper or cardboard production, are removed by the devices. The devices consist of large tanks into which the suspension that is to be supplied to the paper or cardboard machine, is fed in the form of a thin mixture. The suspension is boiled in a vacuum, thereby separating the gas from the tank into a so-called gas chamber, and discharging minor impurities that are present on the liquid surface in the tank. The suspension that is to be supplied to the paper or cardboard machine is removed through an opening in the tank bottom, located there for the purpose of removing suspension that is as gas free as possible from the tank. In order to maximize the degassing capabilities of the device, a vacuum is generated in the tank by way of a vacuum pump, whereby gas escapes through a line from the tank by way of the vacuum pump. In addition, the degassing capability is improved in that the gas-containing suspension is supplied through steel pipes that are located above the liquid level in the tank. This allows the gas that is present in the added suspension, possibly in the form of bubbles, to be discharged prior to the added suspension coming into contact with the suspension already present.
A single overflow serves to stabilize the liquid level in the tank, whereby a partial stream of approximately 1% to 50% of the total volume of suspension that is supplied to the tank, as well as light contaminants that are present on the liquid surface in the tank, flow over the tank's edge. The separated excess suspension, which includes light contaminants, subsequently flows on to further treatment where every effort is made to discharge them from the suspension and to re-introduce the suspension into the loop.
The German Disclosure Document DE 42 34 522 A1 describes a device for degassing a suspension in a deaeration chamber as used in the paper and pulp industry. The separation wall forms the overflow for the separated, excess suspension that is located essentially level with the rear edge of the outlet for the gas-free suspension, vertical to the tank axis. The overflow for the separated, excess suspension itself slopes toward the overflow in the tank, in opposite direction to the flow direction.
Further disclosures regarding the position and form of the separation wall forming the overflow for the separated, excess suspension may be found in the German prior art publications DE 32 19 749 A1, in the PCT application WO 97/15717 and in the US patent of the Applicant U.S. Pat. No. 5,868,905 (PK10542 US).
All of the aforementioned publications referring to the overflow for the separated excess suspension have the following disadvantage in common. The separation wall is positioned vertically and/or symmetrically to the tank axis (longitudinal axis), its location is a given due to its shape in that it is located on one of the two end areas of the tank, therefore its overflow length is a design given.
The Applicant's non-published German prior art patent application DE 200 08 711.8 (PK11075 DE) also discloses a device for degassing suspensions. An overflow in the form of an overflow pipe having an overflow height and an overflow length is located prior to the at least one outlet opening for the separated excess suspension, as viewed in direction of suspension flow. Even though this device permits flexibility with regard to the position and form of the overflow, it does not however permit fluidic optimum positioning of the overflow.